Photodetection device

ABSTRACT

Two light receiving elements are formed on a support substrate. A first light receiving element is formed of a p-type layer, an n-type layer, a light absorption semiconductor layer, an anode electrode, a cathode electrode, a protection film, etc. A second light receiving element is formed of a p-type layer, an n-type layer, a transmissive film, an anode electrode, a cathode electrode, a protection film, etc. The light absorption semiconductor layer absorbs light in a wavelength range λ and disposed closer to the light receiving surface than is the pn junction region. The transmissive film has no light absorption range and disposed closer to the light receiving surface than is the pn junction region. The amount of light in the wavelength range λ is measured through computation using a detection signal from the first light receiving element and a detection signal from the second light receiving element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to: a photodetection device which is adevice having a photoelectric conversion function and configured todetect light of wavelengths in a specific range; and an optical filterused in the photodetection device.

2. Description of the Related Art

Some photodetection devices use what is called a photoconductive sensorelement configured to detect ultraviolet light applied onto a lightreceiving portion thereof on the basis of change in the amount ofphotoinduced current in the light receiving portion. Si semiconductorand the like having detection sensitivity to visible light in awavelength range from 400 nm to 750 nm and the like have conventionallybeen considered for this photoconductive sensor element, for itsinexpensiveness and easiness in controlling doping. The principle ofphotodetection of the photoconductive sensor element is as follows. Asthe semiconductor of the light receiving portion is irradiated withlight having energy equal to or greater than the bandgap, electron-holepairs are created in the semiconductor by a photoelectric conversionfunction, and these carriers are taken out to an external circuit by anexternally applied voltage and detected as an amount of photoinducedcurrent.

Conventional photoelectric conversion elements are generally made of Sias mentioned above. However, since Si is sensitive to all the wavelengthranges shorter than 1.1 μm, it is impossible to take out only light ofspecific wavelengths and measure the amount of that light.

In this respect, a visible light cut filter is generally an interferencefilter in which films with mutually different normal refractive indexesare alternately stacked. However, the cut bandwidth is determined by thedifference between the refractive indexes of the films to be used, andthus, it is difficult to set the entire visible light range from 400 to800 nm as the wavelength range within which the interference filter cancut the light amount down to approximately 0. In addition, theinterference filter inevitably has uncuttable wavelengths, so that theinterference filter may be able to cut visible light but fails to cutinfrared light. In this case, it is difficult to transmit and measureonly ultraviolet light because the Si photoelectric conversion elementis sensitive also to infrared light.

Meanwhile, for the purpose of solving the above problem that occurs dueto the combination of an interference filter and an Si photoelectricconversion element, a configuration has been proposed in which light isdetected while setting mutually different light receiving sensitivitywavelength ranges by making the depths of pn junction interfaces, i.e.,the depths of photoelectric conversion regions, different from eachother. Specifically, one pn junction interface is formed shallow so thatlight can be detected by the photoelectric conversion region havingexcellent sensitivity characteristics to relatively short wavelengthranges. The other pn junction interface is formed deep so that light canbe detected by the photoelectric conversion region having a finesensitivity to long wavelengths. Then, the difference between the twodetected signals is calculated. As a result, short wavelength light canbe detected (see Japanese Patent Application Publication Nos.2009-158570, 2007-67331, 2002-164565, 2009-158928, 2007-305868,2006-318947, for example).

In this case, however, the sensitivity to the ultraviolet range is poor.In addition, the depths of the pn junctions need to be adjusted for eachdetection target wavelength range in the ultraviolet range, which isextremely troublesome. Moreover, it is difficult to detect only light inthe ultraviolet range regardless of how the depths of the pn junctionsare adjusted.

Meanwhile, there is a configuration as described in Patent Document 2 inwhich: the same depth is set for the pn junctions of two photoelectricconversion regions; an ultraviolet light absorption film configured toabsorb part of ultraviolet light is formed on one of photodiodes; andthe difference between the photodiodes is figured out. However, theultraviolet light absorption film is considered as a film configured toabsorb part of ultraviolet light, and as described in the document, is afilm that can only weaken ultraviolet light to be received, by absorbingpart of the ultraviolet light. Since the level of the ultraviolet lightabsorption is weak, the detection sensitivity obtained from thecalculation of the difference is weak as well.

Further, the following problem occurs when the difference between aphotodiode A with an optical filter formed thereon and a photodiode Bprovided with no optical filter is figured out as in Patent Document 2.In the photodiode A with the optical filter formed thereon, interferencefringes are generated as a result of the interference between reflectedlight from the surface of the optical filter and reflected light fromthe interface between the optical filter and the semiconductor layer.Consequently, the photodetection signal comes to contain a signal causedby the interference fringes, making accurate detection impossible.

As described above, it has been difficult to detect measurement-targetlight in a specific wavelength range selectively with a highsensitivity.

SUMMARY OF THE INVENTION

The present invention has been made for solving the above problems, andan object thereof is to provide: a photodetection device capable ofdetecting light in a specific wavelength range selectively with a highsensitivity; and an optical filter used in the photodetection device.

To achieve the above object, the photodetection device of the presentinvention has a main feature which provides a photodetection deviceincluding multiple photodetectors configured to detect light throughphotoelectric conversion and including at least: a first photodetectorincluding a light absorption semiconductor layer at a side closer to alight receiving surface of the first photodetector than is aphotoelectric conversion region of the first photodetector, the lightabsorption semiconductor layer configured to absorb light in awavelength range λ; and a second photodetector including a transmissivefilm at a side closer to a light receiving surface of the secondphotodetector than is a photoelectric conversion region of the secondphotodetector, the transmissive film having no light absorption range.In the photodetection device, an amount of light in the wavelength rangeλ is measured through computation using a signal from the firstphotodetector and a signal from the second photodetector.

Moreover, another configuration of the photodetection device of thepresent invention has a main feature which provides a photodetectiondevice including multiple photodetectors configured to detect lightthrough photoelectric conversion and including at least: a firstphotodetector including a first optical filter at a side closer to alight receiving surface of the first photodetector than is aphotoelectric conversion region of the first photodetector, the firstoptical filter configured to absorb light in a wavelength range λ; and asecond photodetector including a second optical filter at a side closerto a light receiving surface of the second photodetector than is aphotoelectric conversion region of the second photodetector, the secondoptical filter configured to absorb light in a wavelength range λ1including the wavelength range λ or having no light absorption range. Inthe photodetection device, the first and second optical filters areformed such that an interference fringe attributable to thicknesses ofthe filters is not present in a light transmission spectrum, and anamount of light in the wavelength range λ is measured throughcomputation using a signal from the first photodetector and a signalfrom the second photodetector.

Furthermore, the optical filter of the present invention has a mainfeature which provides an optical filter formed by curing a pastesubstance. In the optical filter, the paste substance containssemiconductor particles for absorbing light in a certain wavelengthrange.

The photodetection device of the present invention includes at least:the first photodetector including the light absorption semiconductorlayer at the side closer to the light receiving surface of the firstphotodetector than is the photoelectric conversion region of the firstphotodetector, the light absorption semiconductor layer configured toabsorb light in the wavelength range λ; and the second photodetectorincluding the transmissive film at the side closer to the lightreceiving surface of the second photodetector than is the photoelectricconversion region of the second photodetector, the transmissive filmhaving no light absorption range. Accordingly, light in the wavelengthrange λ can be detected selectively with a high sensitivity, withouthaving to adjust the depth relationship between the two photoelectricconversion regions as in the conventional cases.

Moreover, the photodetection device of the present invention includes atleast: the first photodetector including the first optical filterconfigured to absorb light in the certain wavelength range λ; and thesecond photodetector including the second optical filter configured toabsorb light in the wavelength range λ1 including the wavelength range λor having no light absorption range. Moreover, the first and secondoptical filters are formed such that there is an interference fringeattributable to the thicknesses of the optical filters is not present ina light transmission spectrum; and the amount of the light in thewavelength range λ is measured through computation using the signal fromthe first photodetector and the signal from the second photodetector.Thus, noises due to interference fringes are completely removed.Accordingly, light in a desired wavelength range can be detectedselectively with a high sensitivity.

Moreover, the optical filter of the present invention is the opticalfilter formed by curing the paste substance, and the paste substancecontains the semiconductor particles for absorbing light in the certainwavelength range. Accordingly, the optical filter can be so formed as toabsorb light in a specific wavelength range and to eliminateinterference fringes attributable to the thicknesses of the opticalfilters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an example structure of aphotodetection device of the present invention.

FIG. 2 is a cross-sectional view showing another example structure ofthe photodetection device of the present invention.

FIG. 3 is a cross-sectional view showing still another example structureof the photodetection device of the present invention.

FIGS. 4A to 4E are diagrams showing steps in an experiment fordescribing that when ZnO is used for an ultraviolet light absorptionsemiconductor layer, the reflection of ultraviolet light at the surfaceof the ZnO can be ignored.

FIG. 5 is a graph showing a measurement result obtained from the stepsin the experiment.

FIGS. 6A and 6B are diagrams showing experiment configurations fordescribing that when ZnO is used for the ultraviolet light absorptionsemiconductor layer, ultraviolet light passing through the ZnO can beignored.

FIGS. 7A and 7B are graphs showing measurement results obtained from theconfigurations in FIGS. 6A and 6B.

FIGS. 8A and 8B are diagrams showing structures in which ZnO and SiO₂are formed on PIN PDs, respectively.

FIG. 9 is a graph showing curves of light receiving sensitivity measuredfrom the structures in FIGS. 8A and 8B.

FIG. 10 is a graph showing the light transmission spectrum of each ofZnO/sapphire and SiO₂/sapphire.

FIG. 11 is a graph showing a sensitivity curve calculated by finding thedifference between the sensitivity curves in FIG. 9.

FIG. 12 is a graph showing a state where interference fringes present inthe light receiving sensitivities of the light receiving elements arecaused to coincide with each other by using the ultraviolet lightabsorption semiconductor layer and transmissive film in theconfiguration in FIG. 1

FIG. 13 is a graph showing a sensitivity curve calculated by finding thedifference between the sensitivity curves in FIG. 12.

FIG. 14 is a cross-sectional view showing an example structure which isthe structure in FIG. 1 but in which two photoelectric conversionregions are provided to each light receiving element.

FIG. 15 is a graph showing how light in an ultraviolet range is detectedusing the structure in FIG. 14.

FIG. 16 is a graph showing the correlation between the bandgapequivalent wavelength and the Mg content in MgZnO.

FIG. 17 is a graph showing sensitivity curves of cases where the Mgcontent in Mg_(X)Zn_(1-X)O serving as the ultraviolet light absorptionsemiconductor layer is varied, and of some other cases.

FIG. 18 is a graph showing characteristics of optical filters of thepresent invention.

FIG. 19 is a graph showing characteristics of a differential signal oflight receiving elements using the optical filters of the presentinvention.

FIG. 20 is a cross-sectional view showing the configurations of lightreceiving elements used for the purpose of measuring the characteristicsin FIG. 18.

FIGS. 21A and 21B are diagrams showing a state where an interferencefringe is generated in a light receiving element including aconventional optical filter.

FIG. 22 is a graph showing results of measuring light receivingsensitivities from the configurations in FIG. 21A.

FIG. 23 is a graph showing a differential signal of the two sensitivitycurves in FIG. 22.

FIG. 24 is a cross-sectional view showing the configurations of twolight receiving elements using conventional optical filters and havingthe same optical film thickness.

FIG. 25 is a graph showing results of measuring light receivingsensitivities from the configurations in FIG. 24.

FIG. 26 is a graph showing a differential signal of the two sensitivitycurves in FIG. 25.

FIG. 27 is a cross-sectional view showing an example structure of aphotodetection device using optical filters.

FIG. 28 is a cross-sectional view showing another example structure ofthe photodetection device using optical filters.

FIG. 29 is a cross-sectional view showing still another examplestructure of the photodetection device using optical filters.

FIG. 30 is a graph showing sensitivity curves calculated by finding thedifference between sensitivity curves of the configurations in FIG. 29.

FIG. 31 is a graph showing differential signals obtained by respectivelysubtracting curves PD12 to PD14 in FIG. 30 from a curve PD11 in FIG. 30.

FIG. 32 is a stretched version of FIG. 31.

FIG. 33 is a graph showing curves obtained by finding the sensitivitiesto ranges of ultraviolet lights A, B, and C from the three differentialsignals in FIG. 31.

FIG. 34 is a stretched version of FIG. 33.

FIG. 35 is a graph showing kinds of semiconductors and the absorptionedge wavelength of each semiconductor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinbelow, an embodiment of the present invention will be describedwith reference to the drawings. A photodetection device of the presentinvention is formed of photoelectric conversion elements each having asemiconductor photoelectric conversion layer as its base. Here, thesemiconductor photoelectric conversion layer refers to a semiconductorlayer having a function to convert light into an electric current, andis for example a semiconductor layer having a depletion layer formed ina pn junction or Schottky junction.

First, the photodetection device can be configured as shown in FIG. 1.FIG. 1 is a cross-sectional view showing the structure of thephotodetection device of this embodiment of the present invention. Thephotodetection device includes a shared support substrate 1. Silicon maybe used for the support substrate 1, for example. A light receivingelement 100 and a light receiving element 200 each serving as oneindividual photodetector are formed on the support substrate 1. Thelight receiving elements 100 and 200 are elements configured to detectlight applied from an upper side in the drawing. The polarities p and nmay the reverse of FIG. 1, and the same applies to the following.

In the light receiving element 100, a p-type layer 2 is formed with aninterlayer insulator 10 as a boundary. The p-type layer 2 has an n-typelayer 3 buried in its superficial portion. The n-type layer 3 is formedby doping n-type impurities through the surface of a region of thesuperficial portion situated inward of the periphery of the p-type layer2 by some distance in a plan view. Thus, in the light receiving element100, there is formed a photoelectric conversion region A formed of a pnjunction of the p-type layer 2 and the n-type layer 3. In general, lightreceiving elements are such that, after entering the light receivingsurface, light having a shorter wavelength is absorbed at a shallowerposition.

The surface of each of the p-type layer 2 and the n-type layer 3 iscovered with a transparent protection film 7 made of SiO₂, SiN, or thelike. In addition, the side surface of the p-type layer 2 is coveredwith the interlayer insulator 10. Like the protection film 7, theinterlayer insulator 10 is formed of a transparent film made of SiO₂,SiN, or the like. The protection film 7 has an anode electrode 5 and acathode electrode 6 formed thereon. The anode electrode 5 is connectedto the p-type layer 2 through an opening formed in the protection film7. The cathode electrode 6 is connected to the n-type layer 3 throughanother opening formed in the protection film 7. Thus, a photoelectriccurrent produced by photoelectric conversion in the pn junction regionof the p-type layer 2 and the n-type layer 3 is outputted from thecathode electrode 6 as a photodetection signal.

Meanwhile, the protection film 7 has an ultraviolet light absorptionsemiconductor layer 4 formed thereon in such a manner as to cover thecathode electrode 6. The ultraviolet light absorption semiconductorlayer 4 is a light absorption semiconductor layer configured to absorblight in a wavelength range λ. Specifically, the ultraviolet lightabsorption semiconductor layer 4 is an ultraviolet light absorptionlayer playing a role of an optical filter configured to absorbultraviolet light and transmit light of longer wavelengths than that ofthe ultraviolet light, and is a thin film made of a semiconductor. Theultraviolet range here refers to a wavelength range of 400 nm down toabout 200 nm. This ultraviolet range is further divided into anultraviolet light A (above a wavelength of 320 nm but at or below 400nm), an ultraviolet light B (above a wavelength of 280 nm but at orbelow 320 nm), and an ultraviolet light C (at or below a wavelength of280 nm).

Moreover, the ultraviolet light absorption semiconductor layer 4provided on the light receiving surface side is formed in such a size asto cover the entire photoelectric conversion region A formed of the pnjunction of the p-type layer 2 and the n-type layer 3. The ultravioletlight absorption semiconductor layer 4 is formed to have an area equalto or larger than the area of the photoelectric conversion region A.

For the ultraviolet light absorption semiconductor layer 4, it isdesirable to use a material that selectively absorbs only ultravioletlight. ZnO, MgZnO, TiO₂, SrTiO₂, InGaZnO, and the like are available asoxide materials satisfying the above the requirement. InGaN, AlGaN, GaN,and the like may be used instead. These are materials having a bandgapallowing no adsorption of light in a visible light range, and are highin resistance. In this embodiment, Mg_(X)Zn_(1-X)O (0≦X<1) is used.

On the other hand, in the light receiving element 200, a p-type layer 12on the support substrate 1 has an n-type layer 13 buried in itssuperficial portion. The n-type layer 13 is formed by doping n-typeimpurities through the surface of a region of the superficial portionsituated inward of the periphery of the p-type layer 12 by some distancein a plan view. Thus, in the light receiving element 200, there isformed a photoelectric conversion region B formed of a pn junction ofthe p-type layer 12 and the n-type layer 13.

The pn junctions in the photoelectric conversion regions A and B arecreated at the same depth but may be formed at mutually different depthsinstead. Moreover, it is desirable that each pn junction interfaceshould not be formed at a very deep position, in order to reducecontributions to infrared light as much as possible.

The surface of each of the p-type layer 12 and the n-type layer 13 iscovered with a transparent protection film 17 made of SiO2, SiN, or thelike. In addition, the side surface of the p-type layer 12 is coveredwith the interlayer insulator 10. The protection film 17 has an anodeelectrode 15 and a cathode electrode 16 formed thereon. The anodeelectrode 15 is connected to the p-type layer 12 through an openingformed in the protection film 17. The cathode electrode 16 is connectedto the n-type layer 13 through another opening formed in the protectionfilm 17. Thus, a photoelectric current produced by photoelectricconversion in the pn junction region of the p-type layer 12 and then-type layer 13 is outputted from the cathode electrode 16 as aphotodetection signal.

Meanwhile, the protection film 17 has a transmissive film 14 formedthereon in such a manner as to cover the cathode electrode 16. Thetransmissive film 14 is used as an optical filter having no lightabsorption range. For the transmissive film 14, a dielectric material isused which absorbs no ultraviolet light, is transparent to theultraviolet light and light of longer wavelengths than that of theultraviolet light, and is insulative. SiO₂, ZrO₂, Al₂O₃, Si₃N₄, and thelike are available for the dielectric material used for the transmissivefilm 14. These dielectric materials have a very high transmittance tonot only the ultraviolet light but also visible light to infrared light.In addition, like the transmissive film 14, a film that is transparentto the ultraviolet light and light of longer wavelengths than that ofthe ultraviolet light is desirable for the protection films 7 and 17.Thus, like the transmissive film 14, the protection films 7 and 17 aredesirably made of any of the dielectric materials mentioned above.

Moreover, the transmissive film 14 provided on the light receivingsurface side is formed in such a size as to cover the entirephotoelectric conversion region B formed of the pn junction of thep-type layer 12 and the n-type layer 13. The transmissive film 14 isformed to have an area equal to or larger than the area of thephotoelectric conversion region B.

A method of fabricating the ultraviolet light detection device of FIG. 1will be described. An example fabrication procedure will be describedonly briefly since the device can be created by using a widely knownfabrication technique. An n-type silicon layer is formed on the supportsubstrate 1. The surface (top surface) of the n-type silicon layer isoxidized to form an oxide coating SiO₂, which will become the protectionfilms 7 and 17. Holes are bored through the oxide coating SiO₂, andp-type impurities are introduced therethrough by ion implantation or thelike to create the p-type layers 2 and 12. Next, additional holes arebored through the oxide coating SiO₂, and n-type impurities areintroduced therethrough into a region of each of the p-type layers 2 and12 by ion implantation or the like to create the n-type layers 3 and 13.Meanwhile, the regions of the holes formed in the oxide coating SiO₂will be the regions of the p- and n-type layers which the anode andcathode electrodes 5, 6, 15, and 16 will contact, respectively. Thus,contact regions are formed by ion implantation or the like so that thecontact resistance can be reduced. Thereafter, middle portions and outerportions of the silicon layer are oxidized to create another oxidecoating SiO₂ as the interlayer insulators 10. Next, the anode electrodesand the cathode electrodes are formed by sputtering or vapor deposition,and thereafter the ultraviolet light absorption semiconductor layer 4and the transmissive 14 are formed. Lastly, wiring and the like areperformed.

Next, FIGS. 4A to 4E and FIG. 5 show that use of Mg_(X)Zn_(1-X)O (0≦X<1)for the ultraviolet light absorption semiconductor layer 4 allows thedetection of only ultraviolet light. FIGS. 4A to 4E show how steps in anexperiment are performed. A commercially-available silicon photodiode isused, and photoelectric current outputs obtained by applying ultravioletlight and visible light are compared. First, as shown in FIG. 4A, onlyultraviolet light of a wavelength of 365 nm is applied onto a siliconphotodiode 41 to measure a photoelectric current I_(U).

Next, as shown in FIG. 4B, only visible light is applied onto thesilicon photodiode 41 to measure a photoelectric current I_(V). Next, asshown in FIG. 4C, the 365-nm-wavelength ultraviolet light and thevisible light are applied onto the silicon photodiode 41 to measure aphotoelectric current I_(U+V). As shown in FIG. 4D, a stacked body inwhich a ZnO layer 43 is formed on a glass substrate 42 is disposed onthe silicon photodiode 41. In this state, only the visible light isapplied from above to measure a photoelectric current I1 _(V). Next, asshown in FIG. 4E, the same configuration as FIG. 4D is used, but both ofthe 365-nm-wavelength ultraviolet light and the visible light areapplied to measure a photoelectric current I1 _(U+V).

Here, a reflected component of the visible light at the surface of theZnO layer 43 is calculated. The amount of the reflected component can beexpressed as (I_(V)−I1 _(V)). Meanwhile, it is judged whether or not theultraviolet light is reflected at the surface of the ZnO layer 43. To doso, I_(V) obtained from the measurement of FIG. 4B is subtracted from I1_(U+V) obtained from the measurement of FIG. 4E, and the amount of thereflected component of the visible light (I_(V)−I1 _(V)) at the surfaceof the ZnO layer 43 in FIG. 4D is added thereto. In this way, calculatedis the detection value of only the ultraviolet light in the measurementof FIG. 4E. In short, the calculation is expressed as {I1_(U+V)−I_(V)+(I_(V)−I1 _(V))}. I_(U)={I1 _(U+V)−I_(V)+(I_(V)−I1 _(V))}should be obtained if the ultraviolet light in the measurement of FIG.4E is absorbed with substantially no reflected component at the surfaceof the ZnO layer 43.

FIG. 5 is a graph showing a set of data plotted along a vertical axisrepresenting the photoelectric current I_(U) in the measurement of FIG.4A and a horizontal axis representing {I1 _(U+V)−I_(V)+(I_(V)−I1 _(V))}.The unit is nanoampere (nA) in both the vertical and horizontal axes. Inaddition, measurement and calculation, which are the same as thosedescribed above, are performed while varying the power of the 365-nmwavelength ultraviolet light from 42 μW/cm² to 125 μW/cm², 187 μW/cm²,and 300 μW/cm², and then the obtained set of data is plotted. As can beseen from FIG. 5, a directly proportional straight line is obtained,showing that I_(U)={I1 _(U+V)−I_(V)+(I_(V)−I1 _(V))}.

Next, let us show that an Mg_(X)Zn_(1-X)O layer serving as theultraviolet light absorption semiconductor layer 4 absorbs ultravioletlight substantially completely, and the transmitted components can beignored. Reference sign 41 shown in FIGS. 6A and 6B denotes acommercially-available silicon PIN photodiode. First, the silicon PINphotodiode is packaged as shown in FIG. 6A; both of ultraviolet lightand visible light are applied through a light receiving window 46; andthe resultant photoelectric current is measured. In the measurement, thephotoelectric current is measured first with the output (power density)of the ultraviolet light set at 0 and then the power density isincreased gradually to thereby measure the correlation between thephotoelectric current and the power density of the ultraviolet light. Acurve C1 in FIG. 7A represents the measurement result.

On the other hand, as shown in FIG. 6B, a ZnO layer 43 with a filmthickness of 500 nm is disposed on the top surface of the lightreceiving window 46 of the same package used in the measurement of C1,and both of ultraviolet light and visible light are applied in the samemanner as the measurement of C1. As in the case of C1, the photoelectriccurrent is measured first with the output of the ultraviolet light setat 0 and then the power density is increased gradually. C2 in FIG. 7Arepresents the measurement result.

As C1 in FIG. 7A shows, without the cap of the ZnO layer, thephotoelectric current increases proportionally to the increase in theintensity of the ultraviolet light with the detected electric current ofthe visible light as a base. In contrast, as C2 shows, with the cap ofthe ZnO layer, the photoelectric current substantially keeps a state of0 ultraviolet light output, i.e., the value of the photoelectric currentobtained when only the visible light is detected, regardless of anyincrease in the intensity of the ultraviolet light. FIG. 7B is a graphillustrating a result of subtracting C2 from C1 with the differencebetween their photoelectric currents, which is attributable to thedifference in the visible light output, being adjusted. In FIG. 7B, asubstantially directly proportional straight line is found. FIGS. 7A and7B show that the ZnO layer 43 absorbs the ultraviolet lightsubstantially completely.

Next, FIG. 10 shows that ZnO selectively absorbs only light in theultraviolet range substantially completely. FIG. 10 shows transmittancespectra each covering the wavelengths of light ranging from ultravioletlight to visible light to infrared light in a case where a stacked bodyA and a stacked body B are irradiated with the range of light. Thestacked body A has a ZnO layer formed on a sapphire substrate while thestacked body B has an SiO₂ film formed on the sapphire substrate.Sapphire substrate and SiO₂ are known to be transparent to a wide rangeof light from ultraviolet light to infrared light. Here, thetransmittance spectrum of the SiO₂/sapphire is as shown by a curve inFIG. 10. On the other hand, in the curve of the ZnO/sapphire, thetransmittance decreases abruptly around about a wavelength of 400 nm toa transmittance of 0 and maintains this transmittance of 0 for theultraviolet light.

Measurement is performed using ZnO having the above characteristics.First, as shown in FIG. 8A, a ZnO layer 52 is formed on a silicon PINphotodiode (Si PIN-PD) 51. Assume this as a light receiving element 1.Next, as shown in FIG. 8B, an SiO₂ film 53 is formed on a differentsilicon PIN photodiode 51. Assume this as a light receiving element 2.Then, these light receiving elements are irradiated with light in awavelength range from 200 nm to 1200 nm, and their spectralsensitivities are measured.

FIG. 9 shows curves of the light receiving sensitivities of the lightreceiving elements 1 and 2. The horizontal axis in FIG. 9 represents thewavelength (nm) while the vertical axis in FIG. 9 represents the lightreceiving sensitivity (A/W). The light receiving sensitivity isexpressed as the ratio between the amount of light (watt) incident onthe element and the photoelectric current (ampere) flowing in theelement. A curve denoted by SiO₂ in FIG. 9 corresponds to the lightreceiving element 2 (light receiving element in FIG. 8B) while a curvedenoted by ZnO in FIG. 9 corresponds to the light receiving element 1(light receiving element in FIG. 8A). As can be seen from the spectralsensitivity curves, in the light receiving element 1, the ZnO layer 52absorbs ultraviolet light substantially completely, and therefore thesensitivity in the ultraviolet range is 0. On the other hand, in thelight receiving element 2, the silicon PIN photodiode 51 receives theultraviolet light, and therefore the light in the ultraviolet range isalso detected as photoelectric current outputs.

R in FIG. 11 shows a curve obtained by subtracting the sensitivity curveof the light receiving element 1 in FIG. 9 from the sensitivity curve ofthe light receiving element 2 in FIG. 9. S1 is a spectral sensitivitycurve found as follows. Specifically, the depths of the pn junctions ofthe silicon photodiodes described in the conventional techniques aremade different from each other, and a photoelectric current detected inone pn junction is subtracted from a photoelectric current detected inthe other pn junction. As can be seen by comparing R and S1, themeasurement result of R shows that only the ultraviolet range can bedetected selectively with a very high sensitivity.

Meanwhile, FIG. 1 shows how the above-described light receiving elements1 and 2 are actually aligned and configured. In FIG. 1, the lightreceiving elements 100 and 200 correspond to the light receivingelements 1 and 2, respectively. Moreover, the ultraviolet lightabsorption semiconductor layer 4 corresponds to the ZnO layer 52 in FIG.8A, and the elements below the ultraviolet light absorptionsemiconductor layer 4 correspond to the silicon PIN photodiode 51.Furthermore, the transmissive film 14 corresponds to the SiO₂ film 53 inFIG. 8B, and the elements below the transmissive film 14 correspond tothe silicon PIN photodiode 51.

By subtracting the detection signal of the light receiving element 1from the detection signal of the light receiving element 2 as describedabove, it is possible to extract the detection output of an ultravioletlight component with a very high sensitivity.

Meanwhile, light incident on the ultraviolet light absorptionsemiconductor layer 4 generates a reflected wave at the interfacebetween the ultraviolet light absorption semiconductor layer 4 and theprotection film 7. This reflected wave interferes with a progressivewave, consequently causing an interference fringe in the light receivingelement 100. Moreover, light incident on the transmissive film 14generates a reflected wave at the interface between the transmissivefilm 14 and the protection film 7. This reflected wave interferes with aprogressive wave, consequently causing an interference fringe in thelight receiving element 200. These fringes affect the spectralsensitivity characteristics. FIG. 12 shows spectral sensitivity curvesgenerated corresponding to the interference fringes. As shown in FIG.12, the sensitivity curves wave in a shape corresponding to theinterference fringes.

Here, since the ultraviolet light absorption semiconductor layer 4 ofthe light receiving element 100 and the transmissive film 14 of thelight receiving element 200 are made of different materials, theirrefractive indexes are different. Thus, the pitch, size, and the like ofthe interference fringe differ between the light receiving elements 100and 200. Note that differences in the pitch and size of the interferencefringe affect the sensitivity curve, making accurate measurementimpossible. In this respect, FIG. 12 shows that peaks and valleys influctuations in both sensitivity curves are caused to coincide with eachother by setting the same optical film thickness to both of theultraviolet light absorption semiconductor layer 4 of the lightreceiving element 100 and the transmissive film 14 of the lightreceiving element 200.

Here, the optical film thickness is expressed as filmthickness×refractive index. Accordingly, when N1 and T1 are therefractive index and film thickness of the ultraviolet light absorptionsemiconductor layer 4, respectively, and n and t are the refractiveindex and film thickness of the transmissive film 14, respectively, thefilm thickness T1 of the ultraviolet light absorption semiconductorlayer 4 and the film thickness t of the transmissive film 14 should beadjusted to satisfy N1×T1=n×t.

As shown in FIG. 12, the spectral sensitivity curves coincide with eachother in a wavelength range above 400 nm when the interference fringesof the light receiving elements 100 and 200 coincide with each other.Hence, by subtracting the sensitivity curve P1 of the light receivingelement 100 from the sensitivity curve P2 of the light receiving element200, a photoelectric current can be detected with a very highsensitivity from ultraviolet light of a wavelength of 400 nm or below.Note that an extremely thin semiconductor layer made of the samematerial as the ultraviolet light absorption semiconductor layer 4 maybe formed on the transmissive film 14 of the light receiving element 200to provide the same surface reflection characteristic as those of thelight receiving element 100.

In FIG. 2, a light receiving element 300 is formed additionally in theconfiguration of FIG. 1. This light receiving element 300 is formed tohave the same structure as the light receiving element 100 but has adifferent film thickness for the ultraviolet light absorptionsemiconductor layer. To briefly describe the light receiving element 300serving as a photodetector, a p-type layer 22 is formed on the supportsubstrate 1 with the interlayer insulator 10 as a boundary. The p-typelayer 22 has an n-type layer 23 buried in its superficial portion. Then-type layer 23 is formed by doping n-type impurities through thesurface of a region of the superficial portion situated inward of theperiphery of the p-type layer 22 by some distance. Thus, in the lightreceiving element 300, there is formed a photoelectric conversion regionC formed of a pn junction of the p-type layer 22 and the n-type layer23. Light is converted into an electric current in this photoelectricconversion region C and outputted.

The surface of each of the p-type layer 22 and the n-type layer 23 iscovered with a transparent protection film 27 made of SiO₂, SiN, or thelike. In addition, the side surface of the p-type layer 22 is coveredwith the interlayer insulator 10. The protection film 27 has an anodeelectrode 25 and a cathode electrode 26 formed thereon. The anodeelectrode 25 is connected to the p-type layer 22 through an openingformed in the protection film 27. The cathode electrode 26 is connectedto the n-type layer 23 through another opening formed in the protectionfilm 27. Thus, a photoelectric current produced by photoelectricconversion in the pn junction region of the p-type layer 22 and then-type layer 23 is outputted from the cathode electrode 26 as aphotodetection signal. In addition, the protection film 27 has anultraviolet light absorption semiconductor layer 24 formed thereon insuch a manner as to cover the cathode electrode 26.

The ultraviolet light absorption semiconductor layer 24 is formed insuch a size as to cover the entire photoelectric conversion region Cformed of the pn junction of the p-type layer 22 and the n-type layer23. The ultraviolet light absorption semiconductor layer 24 is formed tohave an area equal to or larger than the area of the photoelectricconversion region C. Here, the ultraviolet light absorptionsemiconductor layer 24 is formed to have a film thickness different fromthat of the ultraviolet light absorption semiconductor layer 4.

Meanwhile, the component of the ultraviolet light absorptionsemiconductor layer 4 of the light receiving element 100 may bedifferent from that of the ultraviolet light absorption semiconductorlayer 24 of the light receiving element 300 in some cases. Since adifferent component of the ultraviolet light absorption semiconductorlayer leads to a different refractive index, the interference fringe aredifferent between the light receiving elements 100 and 300 as mentionedabove. If the interference fringe of the light receiving element 300 isto coincide with those of the light receiving elements 100 and 200, thethicknesses of the ultraviolet light absorption semiconductor layer 4and 24 become different from each other. Then, the optical filmthickness may be determined based on the transmissive film 14. In thiscase, when N2 and T2 are the refractive index and film thickness of theultraviolet light absorption semiconductor layer 24, respectively, and nand t are the refractive index and film thickness of the transmissivefilm 14, respectively, the film thickness T2 of the ultraviolet lightabsorption semiconductor layer 24 should be adjusted to satisfyN2×T2=n×t.

As described above, there will be multiple light receiving elements withultraviolet light absorption semiconductor layers differing from eachother in film thickness if the ultraviolet light absorptionsemiconductor layers differ from each other in refractive index.

Next, the ultraviolet light detection device may be configured as shownin FIG. 3. In FIG. 3, a light receiving element 400 is formedadditionally in the configuration of FIG. 1. This light receivingelement 400 is formed to have the same structure as the light receivingelement 100 but has a light receiving area different from the lightreceiving element 100. To briefly describe the light receiving element400 serving as a photodetector, a p-type layer 32 is formed on thesupport substrate 1 with the interlayer insulator 10 as a boundary. Thep-type layer 32 has an n-type layer 33 buried in its superficialportion. The n-type layer 33 is formed by doping n-type impuritiesthrough the surface of a region of the superficial portion situatedinward of the periphery of the p-type layer 32 by some distance. Thus,in the light receiving element 400, there is formed a photoelectricconversion region D formed of a pn junction of the p-type layer 32 andthe n-type layer 33. Light is converted into an electric current in thisphotoelectric conversion region D and outputted.

The surface of each of the p-type layer 32 and the n-type layer 33 iscovered with a transparent protection film 37 made of SiO₂, SiN, or thelike. In addition, the side surface of the p-type layer 32 is coveredwith the interlayer insulator 10. The protection film 37 has an anodeelectrode 35 and a cathode electrode 36 formed thereon. The anodeelectrode 35 is connected to the p-type layer 32 through an openingformed in the protection film 37. The cathode electrode 36 is connectedto the n-type layer 33 through another opening formed in the protectionfilm 37. Thus, a photoelectric current produced by photoelectricconversion in the pn junction region of the p-type layer 32 and then-type layer 33 is outputted from the cathode electrode 36 as aphotodetection signal. In addition, the protection film 37 has anultraviolet light absorption semiconductor layer 34 formed thereon insuch a manner as to cover the cathode electrode 36.

The ultraviolet light absorption semiconductor layer 34 is formed insuch a size as to cover the entire photoelectric conversion region Dformed of the pn junction of the p-type layer 32 and the n-type layer33. The ultraviolet light absorption semiconductor layer 34 is formed tohave an area equal to or larger than the area of the photoelectricconversion region D. Here, the photoelectric conversion region D in thelight receiving element 400 is formed to have a size (area) differentfrom the size (area) of the photoelectric conversion region A in thelight receiving element 100.

Let us assume that S1 is the area (light receiving area) of thephotoelectric conversion region A in the light receiving element 100 andthat S4 is the area (light receiving area) of the photoelectricconversion region D in the light receiving element 400. The lightreceiving area is the area of the pn junction interface in thisembodiment. Based on a differential signal of the light receivingelements 100 and 400, a detection signal is measured for components oflonger wavelengths than ultraviolet light. In the light receivingelements 100 and 400, their respective ultraviolet light absorptionsemiconductor layers 4 and 34 cut the ultraviolet light. Thus, thedifference in detection photoelectric current between the lightreceiving elements 100 and 400 (I1−I4) is based on incident light oflonger wavelengths than the ultraviolet light, i.e. visible light andinfrared light. When J0 is a photoelectric current induced per unit areaupon incidence of light of longer wavelengths than the ultraviolet lighton the light receiving area S1, J0 is likewise obtained for the lightreceiving area S4 of the light receiving element 400 and can beexpressed as below.(I1−I4)=(S1−S4)×J0

(I1−I4) can be figured out through measurement and calculation.Moreover, the value of (S1−S4) can be figured since it is determinedbased on the design. Thus, J0 can be found easily. Once J0 iscalculated, (J0×S2) is subtracted from (J2×S2) which is the amount ofphotoelectric current of the light receiving element 200 provided withno ultraviolet light absorption semiconductor layer. Here, S2 is thelight receiving area of the light receiving element 200, and J2 is aphotoelectric current produced in the light receiving area S2 per unitarea. Since J2 is a photoelectric current reflecting the result of thedetection of light including the ultraviolet light, visible light, andinfrared light, the above subtraction provides a difference thatrepresents the amount of ultraviolet light; that is, the amount ofultraviolet light={(J2×S2)−(J0×S2)}. S2 may be equal to S1. However, inorder to prevent cancellation of significant digits of a numerical valuein the subtraction computation as much as possible, the followingmeasure may be taken. Multiple combinations of the light receivingelements 100, 200, and 400 are prepared which differ from one another inthe light receiving areas of the light receiving elements includingultraviolet light absorption semiconductor layers. An average value ofthe whole and deviations are calculated for each combination, and theamount of ultraviolet light is finally calculated.

In addition, a visible light cut filter made of a material transparentto ultraviolet light may be formed on the light receiving surface ofeach of the light receiving elements 100, 200, 300, and 400 in FIGS. 1to 3. For example, the visible light cut filter is formed on theultraviolet light absorption semiconductor layers 4, 24, and 34 and thetransmissive film 14. This is done for the purpose of minimizing visiblelight and infrared light having high signal intensities and increasingthe accuracy of the subtraction computation. As the visible light cutfilter, it is possible to use an interference filter in which dielectricfilms having different refractive indexes or the like are alternativestacked, for example.

FIG. 14 shows a photodetection device having a configuration differentfrom those of FIGS. 1 to 3. This photodetection device is formed of alight receiving element 500 and a light receiving element 600 eachserving as photodetector. The photodetection device includes a sharedsupport substrate 61. This support substrate 61 serves also as asubstrate for epitaxial growth, and a high-resistance material isdesirable therefor. Glass may be used, for example.

In the light receiving element 500, an n-type layer 62 has a p-typelayer 63 buried in its superficial portion. The p-type layer 63 isburied by doping p-type impurities through the surface of a region ofthe superficial portion situated inward of the periphery of the n-typelayer 62 by a predetermined distance. Thus, in the light receivingelement 500, there is formed a first photodiode PD31 (firstphotoelectric conversion region) formed of a pn junction of the n-typelayer 62 and the p-type layer 63.

The p-type layer 63 has an n-type layer 64 buried in its superficialportion. The n-type layer 64 is buried by doping n-type impuritiesthrough the surface of a region of the superficial portion situatedinward of the periphery of the p-type layer 63 by some distance. Thus,in the light receiving element 500, there is formed a second photodiodePD32 (second photoelectric conversion region) formed of a pn junction ofthe p-type layer 63 and the n-type layer 64 at a position closer to thelight receiving surface than is the first photodiode PD31. In general,light receiving elements having a photodiode structure are such that,after entering the light receiving surface, light having a shorterwavelength is absorbed at a shallower position. Hence, the secondphotodiode PD32 photoelectrically converts short wavelength light moreefficiently than the first photodiode PD31 does.

The surface of each of the n-type layer 62, the p-type layer 63, and then-type layer 64 is covered with a transparent protection film 65 made ofSiO₂ or SiN. This protection film 65 has a first anode electrode 68, afirst cathode electrode 67, a second anode electrode 70, and a secondcathode electrode 69 formed thereon. The first cathode electrode 67 isconnected to the n-type layer 62 through an opening formed in theprotection film 65. The first anode electrode 68 is connected to thep-type layer 63 through another opening formed in the protection film65. Here, the first anode electrode 68 and the first cathode electrode67 are connected to each other by a wiring formed above the protectionfilm 65. Moreover, the first anode electrode 68 is connected to a groundline (unillustrated), for example.

The second anode electrode 70 is connected to the p-type layer 63through yet another opening formed in the protection film 65. The secondcathode electrode 69 is connected to the n-type layer 64 through stillanother opening formed in the protection film 65. Moreover, the secondanode electrode 70 is as well connected to a ground line(unillustrated), for example. Thus, a photoelectric current produced byphotoelectric conversion in the second photodiode PD32 is outputted fromthe second cathode electrode 69 as a photodetection signal.

The protection film 65 has an ultraviolet light absorption semiconductorlayer 66 formed thereon in such a manner as to cover a region extendingfrom the first anode electrode 68 to the second anode electrode 70. Theultraviolet light absorption semiconductor layer 66 has the samefunction as the ultraviolet light absorption semiconductor layers shownin FIGS. 1 to 3, and is made of the same material. Moreover, theultraviolet light absorption semiconductor layer 66 is formed in such asize as to cover the entire pn junction region of PD31 (firstphotoelectric conversion region). The ultraviolet light absorptionsemiconductor layer 66 is formed to have an area equal to or larger thanthe area of the pn junction region of PD31.

In the light receiving element 500, the pn junction interface of thefirst photodiode PD31 and the pn junction interface of the secondphotodiode PD32 are formed at mutually difference depths.

In the light receiving element 500, the first photodiode PD31 is shortedas the first anode electrode 68 and the first cathode electrode 67 areconnected to each other. Accordingly, of photoelectric currentsoriginating from light incident on the light receiving surface, thephotoelectric current produced by photoelectric conversion in the firstphotodiode PD31 is released to the ground line, and only thephotoelectric current produced by photoelectric conversion in the secondphotodiode PD32 is outputted from the second cathode electrode 69 as aphotodetection signal.

On the other hand, in the light receiving element 600, an n-type layer72 has a p-type layer 73 buried in its superficial portion. The p-typelayer 73 is buried by doping p-type impurities through the surface of aregion of the superficial portion situated inward of the periphery ofthe n-type layer 72 by a predetermined distance. Thus, in the lightreceiving element 600, there is formed a third photodiode PD33 (thirdphotoelectric conversion region) formed of a pn junction of the n-typelayer 72 and the p-type layer 73.

The p-type layer 73 has an n-type layer 74 buried in its superficialportion. The n-type layer 74 is buried by doping n-type impuritiesthrough the surface of a region of the superficial portion situatedinward of the periphery of the p-type layer 73 by some distance. Thus,in the light receiving element 600, there is formed a fourth photodiodePD34 formed of a pn junction of the p-type layer 73 and the n-type layer74 at a position closer to the light receiving surface than is the thirdphotodiode PD33. The fourth photodiode PD34 (forth photoelectricconversion region) photoelectrically converts short wavelength lightmore efficiently than the third photodiode PD33 does.

The surface of each of the n-type layer 72, the p-type layer 73, and then-type layer 74 is covered with a transparent protection film 75 made ofSiO₂ or SiN. This protection film 75 has a third anode electrode 78, athird cathode electrode 77, a fourth anode electrode 80, and a fourthcathode electrode 79 formed thereon. The third cathode electrode 77 isconnected to the n-type layer 72 through an opening formed in theprotection film 75. The third anode electrode 78 is connected to thep-type layer 73 through another opening formed in the protection film75. Here, the third anode electrode 78 and the third cathode electrode77 are connected to each other by a wiring formed above the protectionfilm 75. Moreover, the third anode electrode 78 is connected to a groundline (unillustrated), for example.

The fourth anode electrode 80 is connected to the p-type layer 73through yet another opening formed in the protection film 75. The fourthcathode electrode 79 is connected to the n-type layer 74 through stillanother opening formed in the protection film 75. Moreover, the fourthanode electrode 80 is as well connected to a ground line(unillustrated), for example. Thus, a photoelectric current produced byphotoelectric conversion in the fourth photodiode PD34 is outputted fromthe fourth cathode electrode 79 as a photodetection signal.

The protection film 75 has a transmissive film 76 formed thereon in sucha manner as to cover a region extending from the third anode electrode78 to the fourth anode electrode 80. The transmissive film 76 has thesame function as the transmissive films shown in FIGS. 1 to 3, and ismade of the same material. Moreover, the transmissive film 76 is formedin such a size as to cover the entire pn junction region of PD33 (thirdphotoelectric conversion region). The transmissive film 76 is formed tohave an area equal to or larger than the area of the pn junction regionof PD33.

In the light receiving element 600, the third photodiode PD33 is shortedas the third anode electrode 78 and the third cathode electrode 77 areconnected to each other. Accordingly, of photoelectric currentsoriginating from light incident on the light receiving surface, thephotoelectric current produced by photoelectric conversion in the thirdphotodiode PD33 is released to the ground line, and only thephotoelectric current produced by photoelectric conversion in the fourthphotodiode PD34 is outputted from the fourth cathode electrode 79 as aphotodetection signal.

In the light receiving element 600, the pn junction interface of thethird photodiode PD33 and the pn junction interface of the fourthphotodiode PD34 are formed at mutually different depths. Moreover, thepn junction interface of PD31 in the light receiving element 500 and thepn junction interface of PD33 in the light receiving element 600 areformed at mutually different depths. Furthermore, the pn junctioninterface of PD32 in the light receiving element 500 and the pn junctioninterface of PD34 in the light receiving element 600 are formed at thesame depth.

The second and forth photodiodes PD32 and PD34 are each formed at such adepth that light in a wavelength range of for example 400 nm to 600 nmcan be photoelectrically converted most efficiently. Moreover, the firstand third photodiodes PD31 and PD33 are each formed at such a depth thatlight in a wavelength range of for example 600 nm to 800 nm can bephotoelectrically converted most efficiently.

FIG. 15 shows light receiving sensitivity curves of the ultravioletlight detection device in FIG. 14. First, a light receiving sensitivitycurve that is based on a detection signal obtained by applying lightranging from ultraviolet light to infrared light appears as R1 when thethird photodiode PD33 is not shorted in the light receiving element 600.Here, J1 indicates a wavelength which varies depending on the depth ofthe pn junction interface of each of PD31 and PD33 and corresponds to adepth at which photoelectric conversion is done most efficiently.

An output SP1 on a long wavelength side disappears when the measurementis performed with the third photodiode PD33 shorted as in theconfiguration of the light receiving element 600. Thus, the sensitivitycurve appears as R2. Next, in the case of the light receiving element500, since the first photodiode PD31 is shorted, the application of thelight ranging from ultraviolet light to infrared light provides a lightreceiving sensitivity curve that appears in the shape of R2 as in thecase of the light receiving element 600. However, the sensitivity curveappears as R3 which is R2 with a sensitivity ABS in the ultravioletrange removed therefrom, because the ultraviolet light absorptionsemiconductor layer 66 absorbs the ultraviolet light substantiallycompletely.

So, the light receiving sensitivity curve R3 can be obtained from thelight receiving element 500, and the light receiving sensitivity curveR2 can be obtained from the light receiving element 600. Then, thesensitivity ABS in the ultraviolet range can be figured out by findingthe difference between the light receiving sensitivity curves obtainedfrom the respective light receiving elements 500 and 600; that is,ABS=(R2−R3). Through the above procedure, ultraviolet light can bedetected.

FIG. 16 is a graph showing the correlation between the value of X inMg_(X)Zn_(1-X)O and a bandgap equivalent wavelength (nm) relative to theMg content in a case where Mg_(X)Zn_(1-X)O is used for the ultravioletlight absorption semiconductor layer. The bandgap equivalent wavelengthis related to the absorption wavelength point (nm) of the semiconductor.The larger the value of X, the shorter the absorption wavelength ofMg_(X)Zn_(1-X)O. As can be seen from the graph, the light receivingsensitivity range of the light receiving element can be changed bychanging the Mg content X in Mg_(X)Zn_(1-X)O.

FIG. 17 is a graph showing: sensitivity curves of cases where the valueof X in Mg_(X)Zn_(1-X)O serving as the ultraviolet light absorptionsemiconductor layer in the configuration of FIG. 1 is varied on thebasis of the correlation in FIG. 16; and a sensitivity curve of a casewhere AlGaN is used for the ultraviolet light absorption semiconductorlayer. As can be seen from FIG. 16, as the Mg content X increases, thebandgap equivalent wavelength becomes shorter, which in turn shifts therange of the light receiving sensitivity curve toward its shorterwavelength side and narrows the width of the sensitivity curve.

For this reason, multiple light receiving elements may be prepared ineach of which the ultraviolet light absorption semiconductor layer usesMg_(X)Zn_(1-X)O having a different Mg content X from those of theothers. Moreover, a light receiving element may be prepared in which atransmissive film is formed instead of the ultraviolet light absorptionsemiconductor layer. Then, the difference between them may be figuredout. In this way, it is possible to calculate the light receivingsensitivity in each of the wavelength ranges of the ultraviolet light A(above a wavelength of 320 nm but at or below 400 nm), the ultravioletlight B (above a wavelength of 280 nm but at or below 320 nm), and theultraviolet light C (at or below a wavelength of 280 nm). S1 and S2 arespectral sensitivity curves obtained by setting mutually differentdepths for the pn junctions of the silicon photodiodes described in theconventional techniques, and subtracting a photoelectric currentdetected in one pn junction from that detected in the other. Asillustrated, the light receiving sensitivity is significantly improvedas compared to the conventional cases, and only ultraviolet light can bedetected. Moreover, as can be seen from the graph, it is possible to useAlGaN or the like.

Next, optical filters of the present invention will be described. Asshown in FIG. 20, commercially available photodiodes are used which arethe same except for their optical filters 101A and 101B. Each of thephotodiodes is a pn junction photodiode made of silicon (Si). In PD1, ap-type Si semiconductor 152 is formed on an n-type Si semiconductor 151,and the optical filter 101A is formed on the p-type Si semiconductor152. On the other hand, in PD2, a p-type Si semiconductor 152 is formedon an n-type Si semiconductor 151, and the optical filter 101B is formedon the p-type Si semiconductor 152.

The optical filter 101A is formed by curing a paste substance. In theexample of FIG. 20, a glass paste is used. The optical filter 101A madeof the glass paste is configured to transmit ultraviolet light, visiblelight, infrared light, and the like and is of a material configured notto absorb light of any specific wavelength. On the other hand, theoptical filter 101B is formed by curing a paste substance and is made ofa glass paste in which semiconductor particles configured to absorblight of specific wavelengths are mixed. In the example of FIG. 20, inthe optical filter 101B, ZnO particles are mixed in the glass paste. ZnO(zinc oxide) is a material that can play a role of an optical filterconfigured to absorb ultraviolet light and transmit light with longerwavelengths than that of the ultraviolet light.

PD1 and PD2 in FIG. 20 are irradiated with light including ultravioletlight from above, and their light receiving sensitivities are measured.The measurement results are shown in FIG. 18. FIG. 18 shows spectralsensitivity curves of PD1 and PD2. The horizontal axis in FIG. 18represents the wavelength (nm) while the vertical axis in FIG. 18represents the light receiving sensitivity. In general, light receivingsensitivity is expressed as the ratio between the amount of light (watt)incident on the element and the photoelectric current (ampere) flowingin the element. In FIG. 20, however, an arbitrary unit is employed byperforming normalization using the highest sensitivity value or thelike.

A curve denoted by X1 in FIG. 18 corresponds to the light receivingelement PD1 while a curve denoted by X2 in FIG. 18 corresponds to thelight receiving element PD2. As can be seen from the spectralsensitivity curves, in PD2, the optical filter 101B made of the glasspaste containing ZnO absorbs ultraviolet light substantially completely,and therefore the sensitivity in the ultraviolet range is 0. On theother hand, in PD1, the optical filter 101A transmits the ultravioletlight and the pn junction portion of the photodiode receives theultraviolet light, and therefore the light in the ultraviolet range isdetected as a photoelectric current output.

Here, fluctuations attributable to interference fringes are not found inthe light receiving sensitivity in either the curve X1 or X2. Thus, bysubtracting the sensitivity curve X2 from the sensitivity curve X1, asensitivity curve X3 shown in FIG. 19 is obtained as a differentialsignal; that is, X3=X1−X2. As can be seen from FIG. 19, the sensitivitycurve X3 shows sensitivity in the ultraviolet light range andsubstantially 0 sensitivity in the other wavelength ranges. Moreover, adetection signal affected by interference fringes is not present.

To compare problems related to interference fringe, a light receivingelement PD3 and a light receiving element PD4 in FIG. 21A each using thesame photodiode as those in FIG. 20 are used, and their light receivingsensitivities are measured. No optical filter is formed on thephotodiode of the light receiving element PD3, while an optical filter153 made of a ZnO film formed by sputtering is formed on the photodiodeof the light receiving element PD4. The measurement is performed byirradiating PD3 and PD4 with light including ultraviolet light fromabove.

The measurement results are shown in FIG. 22. The horizontal axis inFIG. 22 represents the wavelength (nm) while the vertical axis in FIG.22 represents the light receiving sensitivity (arbitrary unit). A curve(dotted line) denoted by F1 in FIG. 22 corresponds to the lightreceiving element PD3 while a curve (solid line) denoted by F2 in FIG.22 corresponds to the light receiving element PD4. As can be seen fromthe spectral sensitivity curves, in PD3, ultraviolet light as well aslight in the visible light range and infrared light range are alldetected. Moreover, cyclic signal fluctuations which seem to originatefrom interference fringes are not present.

On the other hand, in PD4, the optical filter 153 made of a ZnO filmabsorbs the ultraviolet light substantially completely, and thereforethe sensitivity in the ultraviolet range is 0. However, in the curve F2,cyclic fluctuations are found in the light receiving sensitivityspectrum over a wide range, implying an influence of interferencefringes.

This can be explained using FIG. 21B. When incident on the opticalfilter 153 formed of a ZnO film, light including ultraviolet lightgenerates a reflected wave at the boundary between the atmosphere andthe optical filter 153. In addition, the light having entered theoptical filter 153 generates a reflected wave at the interface betweenthe optical filter 153 and the p-type Si semiconductor layer 152. Thesetwo reflected waves interfere with each other and cause an interferencefringe. This interference fringe influences the spectral sensitivitycharacteristics. Interference fringe is generated at a certain cycle,and therefore the fluctuations appear in the spectral sensitivity curvecyclically as well. Thus, the sensitivity curve F2 waves in a shapecorresponding to interference fringes.

FIG. 23 shows a sensitivity curve being a differential signal obtainedby subtracting the curve F2 from the curve F1 (F1−F2). As can be seenfrom the differential signal, the curve swings in the positive andnegative directions in phase with the cyclic fluctuations attributableto the interference fringes. Moreover, in FIG. 23, the signalfluctuating due to the interference fringes are significantly greaterthan the detection signal of the ultraviolet light. Thus, theultraviolet light cannot be accurately detected at all.

Next, a ZrO₂ film having the same optical film thickness as the opticalfilter 153 of the light receiving element PD4 in FIG. 21A is stacked asan optical filter 154 on the light receiving element PD3 in FIG. 21A,and their light receiving sensitivities are measured. Specifically, alight receiving element PD5 is used in which an optical filter 154formed of a ZrO₂ film formed by sputtering is stacked on the photodiodeas shown in FIG. 24. This ZrO₂ film is configured to transmit lightranging from ultraviolet light to visible light to infrared light.Meanwhile, PD6 is the same as PD4 in FIGS. 21A and 21B. Here, theoptical film thickness is expressed as film thickness×refractive index.The optical filter 153 and the optical filter 154 are prepared such thatN1×T1=n×t can be satisfied, where N1 and T1 are the refractive index andfilm thickness of the optical filter 153 formed of the ZnO film,respectively, and n and t are the refractive index and film thickness ofthe optical filter 154 formed of the ZrO₂ film, respectively.

DP5 and PD6 thus prepared are irradiated with light includingultraviolet light from above, and their light receiving sensitivitiesare measured. The measurement results are shown in FIG. 25. Thehorizontal axis in FIG. 25 represents the wavelength (nm) while thevertical axis in FIG. 25 represents the light receiving sensitivity(arbitrary unit). A curve (dotted line) denoted by X3 in FIG. 25corresponds to the light receiving element PD5 while a curve (solidline) denoted by X4 in FIG. 25 corresponds to the light receivingelement PD6. In PD5, ultraviolet light as well as light in the visiblelight range and infrared light range are all detected. On the otherhand, in PD6, the ultraviolet light is absorbed, and therefore thesensitivity to the ultraviolet light is 0. Moreover, as can be seen fromthe spectral sensitivity curves, cyclic fluctuations due to interferencefringes are present in both of the curves X3 and X4.

Since the optical filters 153 and 154 have the same optical filmthickness, there is a similarity therebetween in the cycle and size ofthe fluctuations in the visible light range attributable to theinterference fringes. However, in the infrared light range, there aregaps in the degree of the light receiving sensitivity between X3 and X4as a range D shows, so that the sensitivities cannot match each other.This is because the wavelength dispersion of refractive index variesdepending on the material from which the optical filter is prepared, andthis makes it impossible to make the sensitivities match each otheracross all the wavelength ranges. For example, if interference fringesin a blue light range are to coincide with each other, there will begaps in the sensitivity in the infrared light range.

FIG. 26 shows a sensitivity curve being a differential signal obtainedby subtracting the curve X4 from the curve X3 (X3−X4). As can be seenfrom this differential signal, the detection signal appears relativelylarge in the ultraviolet light range, but the detection signal appearsstill large in the infrared light range. Thus, the ultraviolet lightcannot be detected accurately.

As described above, using optical filters formed on photodiodes causesthe above interference fringe problem and makes it difficult to detectlight of specific wavelengths accurately. However, using the opticalfilters 101A and 101B in FIG. 20 makes it possible to obtain a signalcompletely free from noises attributable to interference fringes, as thesignal in FIG. 19 shows. Accordingly, it is possible to detect light ofspecific wavelengths accurately with a high sensitivity.

To eliminate interference fringes, light scattering inside the opticalfilters is considered necessary. For this reason, optical filtersobtained by curing a paste substance are used. A glass paste can beparticularly easily formed into a thick film. Moreover, use of a pastesubstance permits the occurrence of light scattering and thereforeprevents the generation of interference fringes. Meanwhile, thegeneration of interference fringes can be prevented even when ZnO powderis added to the paste substance. Also, it is desirable that scatteringby ZnO be smaller than scattering by the glass paste.

Here, the paste substance may be any material as long as it transmits awide range of light from ultraviolet light to infrared light. Forexample, it is possible to use an acrylic resin, an amorphousfluororesin (amorphous fluoropolymer), a silicone resin, afluorine-based resin, a glass, or the like. It is particularlypreferable that the paste substance have a thermal expansion coefficientclose to those of the semiconductors or substrate on which the opticalfilters 101A and 101B are stacked, because such a configuration makesthe paste substance less likely to be peeled off.

In addition, the optical filters 101A and 101B are preferably formed tohave, but not particularly limited to, a film thickness of about 0.1 μmto 5 μm if there is a large difference in thermal expansion coefficientbetween the optical filters 101A and 101B and the semiconductors orsubstrate on which the optical filters 101A and 101B are stacked.Further, if the optical filters are to be formed by applying the pastesubstance on the semiconductors or substrate, a low-melting material isdesirably used for the purpose of reducing damage on the semiconductorsor substrate. From the viewpoint of thermal expansion coefficient andmelting point, the major component of the paste is desirably a glassmaterial, for example, as described above.

Moreover, one of the optical filters is prepared by curing a materialhaving the paste substance as its major component and semiconductorparticles added thereto. In this case, the semiconductor particles areundesirably a semiconductor powder having such a particle size as towhiten the paste substance when added thereto. This is because such apowder makes it difficult for the optical filter to transmit not onlyultraviolet light but also visible light and the like, which in turnreduces the amount of light reaching the depletion layer formed at theinterface between the p-type Si semiconductor 152 and the n-type Sisemiconductor 151 and therefore makes the light detection impossible.

Next, a method of fabricating the optical filter 101B will be described.As the ZnO powder, 9 g of a ZnO powder having a particle size of 100 nmis used. As the major component of the paste, 85 g of a glass paste isused. 15 g of a dilution oil is mixed into the ZnO powder and the glasspaste to thereby prepare a ZnO-powder-containing glass paste. Theviscosity of this glass paste falls within a normal range and is 0.1 to500000 mPas, for example. The dilution oil is used to adjust theviscosity, and the ratio of the dilution oil may be any ratio as long asa desired transmittance can be obtained eventually. TheZnO-powder-containing glass paste is screen-printed on the p-type Sisemiconductor 152. As a result, the light receiving element PD2 isformed. As the method of forming the optical filter 101A or 101B, spincoating, dipping, or the like may be used to form the optical filter101A or 101B, instead of the screen printing mentioned above.

Meanwhile, for the optical filter 101A, only the glass paste isscreen-printed on the p-type Si semiconductor 152 so that the opticalfilter 101A would not contain the ZnO powder. Both of the opticalfilters 101A and 101B are prepared and burned in the same manner so asto provide them with the same transparency. This eliminates a differencein the optical transmittance involving scattering and thus eliminates adifference in the light receiving sensitivity.

The ultraviolet range here refers to a wavelength range from 400 nm downto about 200 nm. This ultraviolet range is further divided into theultraviolet light A (above a wavelength of 320 nm but at or below 400nm), the ultraviolet light B (above a wavelength of 280 nm but at orbelow 320 nm), and the ultraviolet light C (at or below a wavelength of280 nm).

In a case where the glass paste is used as the major component of thepaste substance as described above, the glass paste absorbs thewavelengths in the ranges of the ultraviolet light B and below, whereasthe ZnO absorbs the wavelengths in the whole ultraviolet range. Thus,formed is a photodetection device which detects only the ultravioletlight A as shown in FIG. 19 by finding the difference between the lightreceiving elements PD1 and PD2.

FIG. 27 shows an example where a photodetection device is configuredusing the optical filters described above. This photodetection devicehas basically the same structure as the photodetection device shown inFIG. 1. Since components in FIG. 27 denoted by the same reference signsas those in FIG. 1 have the same configurations, description thereofwill be omitted. Note that a shared support substrate 40 plays the samerole as the support substrate 1 in FIG. 1, and is made of silicon, forexample.

The difference from FIG. 1 is that an optical filter 4A is formed on theprotection film 7, and an optical filter 14A is formed on the protectionfilm 17. The optical filter 4A is equivalent to a light absorption layerformed by curing a paste substance containing no semiconductor particlesor a paste substance containing semiconductor particles, and configuredto absorb light in a specific wavelength range. Alternatively, theoptical filter 4A may be a dummy layer configured not to absorb light inany specific wavelength range. In one example, the optical filter 101Ain FIG. 20 may serve as the optical filter 4A.

Moreover, the optical filter 4A provided on the light receiving surfaceside is formed in such a size as to cover the entire photoelectricconversion region A formed of the pn junction of the p-type layer 2 andthe n-type layer 3. The optical filter 4A is formed to have an areaequal to or larger than the area of the photoelectric conversion regionA.

On the other hand, the optical filter 14A is equivalent to a lightabsorption layer formed by curing a paste substance containing nosemiconductor particles or a paste substance containing semiconductorparticles, and configured to absorb light in a specific wavelengthrange. Alternatively, the optical filter 14A may be a dummy layerconfigured not to absorb light in any specific wavelength range. In oneexample, the optical filter 101B in FIG. 20 may serve as the opticalfilter 14A.

Moreover, the optical filter 14A provided on the light receiving surfaceside is formed in such a size as to cover the entire photoelectricconversion region B formed of the pn junction of the p-type layer 12 andthe n-type layer 13. The optical filter 14A is formed to have an areaequal to or larger than the area of the photoelectric conversion regionB.

A method of fabricating the photodetection device in FIG. 27 will bedescribed. An example fabrication procedure will be described onlybriefly since the device can be created by using a widely knownfabrication technique. An n-type silicon layer is formed on the supportsubstrate 40. The surface (top surface) of the n-type silicon layer isoxidized to form an oxide coating SiO₂, which will become the protectionfilms 7 and 17. Holes are bored through the oxide coating SiO₂, andp-type impurities are introduced therethrough by ion implantation or thelike to create the p-type layers 2 and 12.

Next, additional holes are bored through the oxide coating SiO₂, andn-type impurities are introduced therethrough into the p-type layers 2and 12 by ion implantation or the like to create the n-type layers 3 and13. Meanwhile, the regions of the holes formed in the oxide coating SiO₂will be the regions of the p- and n-type layers which the anode andcathode electrodes 5, 6, 15, and 16 will contact, respectively. Thus,contact regions are formed by ion implantation or the like so that thecontact resistance can be reduced. Thereafter, middle portions and outerportions of the silicon layer are oxidized to create another oxidecoating SiO₂ as the interlayer insulators 10. Next, the anode electrodesand the cathode electrodes are formed by sputtering or vapor deposition,and thereafter the optical filters 4A and 14A are formed. Lastly, wiringand the like are performed.

Next, the photodetection device can be configured as shown in FIG. 29.In FIG. 29, a light receiving element 400 is formed additionally in theconfiguration of FIG. 27. This light receiving element 400 is formed tohave the same structure as the light receiving element 100 but has alight receiving area different from the light receiving element 100. Tobriefly describe the light receiving element 400 serving as aphotodetector, a p-type layer 32 is formed on the support substrate 40with the interlayer insulator 10 as a boundary. The p-type layer 32 hasan n-type layer 33 buried in its superficial portion. The n-type layer33 is formed by doping n-type impurities through the surface of a regionof the superficial portion situated inward of the periphery of thep-type layer 32 by some distance. Thus, in the light receiving element400, there is formed a photoelectric conversion region D formed of a pnjunction of the p-type layer 32 and the n-type layer 33. Light isconverted into an electric current in this photoelectric conversionregion D and outputted.

The surface of each of the p-type layer 32 and the n-type layer 33 iscovered with a transparent protection film 37 made of SiO₂, SiN, or thelike. In addition, the side surface of the p-type layer 32 is coveredwith the interlayer insulator 10. The protection film 37 has an anodeelectrode 35 and a cathode electrode 36 formed thereon. The anodeelectrode 35 is connected to the p-type layer 32 through an openingformed in the protection film 37. The cathode electrode 36 is connectedto the n-type layer 33 through another opening formed in the protectionfilm 37. Thus, a photoelectric current produced by photoelectricconversion in the pn junction region of the p-type layer 32 and then-type layer 33 is outputted from the cathode electrode 36 as aphotodetection signal. In addition, the protection film 37 has anoptical filter 34A formed thereon in such a manner as to cover thecathode electrode 36.

The optical filter 34A is equivalent to a light absorption layer formedby curing a paste substance containing no semiconductor particles or apaste substance containing semiconductor particles, and configured toabsorb light in a specific wavelength range. Moreover, the opticalfilter 34A is formed of a light absorption layer made of the samematerial as the optical filter 4A of the light receiving element 100 andconfigured to absorb light in a certain wavelength range λ (a range froma lower-limit wavelength λL to an upper-limit wavelength λU). This meansthat the optical filter 4A is likewise formed of the light absorptionlayer configured to absorb light in the wavelength range λ (the rangefrom the lower-limit wavelength λL to the upper-limit wavelength λU).

On the other hand, the optical filter 14A of the light receiving element200 is formed by curing a paste of an amorphous fluororesin or the likehaving a very high transmittance to not only ultraviolet light but alsovisible light to infrared light. Here, a range from ultraviolet light toinfrared light (including visible light) assumes a wavelength range from200 nm to 1200 nm as shown in FIG. 18 and the like.

The optical filter 34A is formed in such a size as to cover the entirephotoelectric conversion region D formed of the pn junction of thep-type layer 32 and the n-type layer 33. The optical filter 34A isformed to have an area equal to or larger than the area of thephotoelectric conversion region D. Here, the photoelectric conversionregion D in the light receiving element 400 is formed to have a size(area) different from the size (area) of the photoelectric conversionregion A in the light receiving element 100.

Let us assume that S1 is the area (light receiving area) of thephotoelectric conversion region A in the light receiving element 100 andthat S4 is the area (light receiving area) of the photoelectricconversion region D in the light receiving element 400. The lightreceiving area is the area of the pn junction interface in thisembodiment. Based on a differential signal of the light receivingelements 100 and 400, a detection signal is measured for a wavelengthrange λ0 which is a wavelength range of ultraviolet light to infraredlight and excludes the wavelength range λ. In the light receivingelements 100 and 400, their respective optical filters 4A and 34A cutlight in the wavelength range λ0. Thus, the difference in detectionphotoelectric current between the light receiving elements 100 and 400(I1−I4) is based on light in the wavelength range λ0 being theultraviolet-infrared wavelength range excluding the wavelength range λ.When J0 is a photoelectric current induced per unit area upon incidenceof light in the wavelength range λ0 on the light receiving area S1, J0is likewise obtained for the light receiving area S4 of the lightreceiving element 400 and can be expressed as below:(I1−I4)=(S1−S4)×J0

(I1−I4) can be figured out through measurement and calculation.Moreover, the value of (S1−S4) can be figured since it is determinedbased on the design. Thus, J0 can be found easily. Once J0 iscalculated, (J0×S2) is subtracted from (J2×S2) which is the amount ofphotoelectric current of the light receiving element 200 having noabsorption range in the ultraviolet-infrared light range. Here, S2 isthe light receiving area of the light receiving element 200, and J2 is aphotoelectric current produced in the light receiving area S2 per unitarea. Since J2 is a photoelectric current reflecting the result ofdetection of light including ultraviolet light, visible light, andinfrared light, the above subtraction provides a difference thatrepresents the amount of light in the wavelength range λ0; that is, theamount of light in the wavelength range λ0={ (J2×S2)−(J0×S2)}. S2 may beequal to S1. However, in order to prevent cancellation of significantdigits of a numerical value in the subtraction computation as much aspossible, the following measure may be taken. Multiple combinations ofthe light receiving elements 100, 200, and 400 are prepared which differfrom one another in the light receiving areas of the light receivingelements including optical filters that absorb light in the wavelengthrange λ. An average value of the whole and deviations are calculated foreach combination, and the amount of light in the wavelength range λ isfinally calculated.

Next, the photodetection device can be configured as shown in FIG. 28.FIG. 28 shows a photodetection device configured by four light receivingelements obtained by adding, to the configuration in FIG. 27, lightreceiving elements 300 and 500 having the same structures as the lightreceiving elements 100 and 200. Note that optical filters 24 and 44 ofthe light receiving elements 300 and 500 are formed to have lightabsorption wavelength ranges different from those of the optical filters4A and 14A of the light receiving elements 100 and 200.

To briefly describe the light receiving element 300 serving as aphotodetector, a p-type layer 22 is formed on the support substrate 40with the interlayer insulator 10 as a boundary. The p-type layer 22 hasan n-type layer 23 buried in its superficial portion. The n-type layer23 is formed by doping n-type impurities through the surface of a regionof the superficial portion situated inward of the periphery of thep-type layer 22 by some distance. Thus, in the light receiving element300, there is formed a photoelectric conversion region C formed of a pnjunction of the p-type layer 22 and the n-type layer 23. Light isconverted into an electric current in this photoelectric conversionregion C and outputted.

The surface of each of the p-type layer 22 and the n-type layer 23 iscovered with a transparent protection film 27 made of SiO₂, SiN, or thelike. In addition, the side surface of the p-type layer 22 is coveredwith the interlayer insulator 10. The protection film 27 has an anodeelectrode 25 and a cathode electrode 26 formed thereon. The anodeelectrode 25 is connected to the p-type layer 22 through an openingformed in the protection film 27. The cathode electrode 26 is connectedto the n-type layer 23 through another opening formed in the protectionfilm 27. Thus, a photoelectric current produced by photoelectricconversion in the pn junction region of the p-type layer 22 and then-type layer 23 is outputted from the cathode electrode 26 as aphotodetection signal. In addition, the protection film 27 has theoptical filter 24 formed thereon in such a manner as to cover thecathode electrode 26.

The optical filter 24 is equivalent to a light absorption layer formedby curing a paste substance containing no semiconductor particles or apaste substance containing semiconductor particles, and configured toabsorb light in a specific wavelength range.

To briefly describe the light receiving element 500 serving as aphotodetector, a p-type layer 42 is likewise formed on the supportsubstrate 40 with the interlayer insulator 10 as a boundary. The p-typelayer 42 has an n-type layer 43 buried in its superficial portion. Then-type layer 43 is formed by doping n-type impurities through thesurface of a region of the superficial portion situated inward of theperiphery of the p-type layer 42 by some distance. Thus, in the lightreceiving element 500, there is formed a photoelectric conversion regionE formed of a pn junction of the p-type layer 42 and the n-type layer43. Light is converted into an electric current in this photoelectricconversion region E and outputted.

The surface of each of the p-type layer 42 and the n-type layer 43 iscovered with a transparent protection film 47 made of SiO₂, SiN, or thelike. In addition, the side surface of the p-type layer 42 is coveredwith the interlayer insulator 10. The protection film 47 has an anodeelectrode 45 and a cathode electrode 46 formed thereon. The anodeelectrode 45 is connected to the p-type layer 42 through an openingformed in the protection film 47. The cathode electrode 46 is connectedto the n-type layer 43 through another opening formed in the protectionfilm 47. Thus, a photoelectric current produced by photoelectricconversion in the pn junction region of the p-type layer 42 and then-type layer 43 is outputted from the cathode electrode 46 as aphotodetection signal. In addition, the protection film 47 has theoptical filter 44 formed thereon in such a manner as to cover thecathode electrode 46.

The optical filter 44 is equivalent to a light absorption layer formedby curing a paste substance containing no semiconductor particles or apaste substance containing semiconductor particles, and configured toabsorb light in a specific wavelength range.

Here, the optical filters 4A, 14A, 24, and 44 are formed as below. Theoptical filter 4A is formed by curing a paste containing only anamorphous fluororesin (amorphous fluoropolymer). The optical filter 14Ais formed by curing an amorphous fluororesin paste containing Ga₂O₃particles. The optical filter 24 is formed by curing an amorphousfluororesin paste containing MgZnO particles. The optical filter 44 isformed by curing an amorphous fluororesin paste containing ZnOparticles.

FIG. 35 shows the absorption edge wavelengths (nm) of semiconductorparticles that can be used for the optical filters. The horizontal axisrepresents the kind of semiconductor element or compound to be added tothe optical filters while the vertical axis represents the absorptionedge wavelength, i.e. bandgap equivalent wavelength (nm). As can be seenfrom the graph, Ga₂O₃ absorbs the wavelengths of the ultraviolet light C(wavelengths of 280 nm and below). ZnO absorbs all the ultravioletlights (ultraviolet lights A+B+C: 400 nm and below). Meanwhile,Mg_(X)Zn_(1-X)O (0≦X<1) is known such that its absorption edgewavelength shifts toward the shorter wavelength side in the ultravioletlight range as the Mg content X increases. Thus, the light receivingsensitivity range of the light receiving element can be changed bychanging the Mg content X in Mg_(X)Zn_(1-X)O.

In this embodiment, X is set at 0.3, so that the optical filter 24 ismade of an amorphous fluororesin containing particles ofMg_(0.3)Zn_(0.7)O and absorbs the ultraviolet lights B and C (320 nm andbelow).

Light of 200 nm to 1200 nm is applied from an upper side of thephotodetection device in FIG. 28, and spectral sensitivities aremeasured. FIG. 30 shows light receiving sensitivity curves of the lightreceiving elements 100 (PD11), 200 (PD12), 300 (PD13), and 500 (PD14).The horizontal axis represents the wavelength (nm) while the verticalaxis represents the light receiving sensitivity (arbitrary unit). Thesensitivity curve PD11 shows sensitivity across the range from 200 nm to1200 nm since the optical filter 4A of the light receiving element 100transmits light ranging from ultraviolet light to infrared light. On theother hand, the sensitivity curve PD12 appears as a curve showing nosensitivity to the wavelengths of the ultraviolet light C since theoptical filter 14A of the light receiving element 200 absorbs thewavelengths of the ultraviolet light C.

Meanwhile, the sensitivity curve PD13 appears as a curve showing nosensitivity to the wavelengths of the ultraviolet lights B and C sincethe optical filter 24 of the light receiving element 300 absorbs thewavelengths of the ultraviolet lights B and C. Moreover, the sensitivitycurve PD14 appears as a curve showing no sensitivity to almost all theultraviolet lights since the optical filter 44 of the light receivingelement 500 absorbs all the ultraviolet lights.

FIG. 31 shows signals corresponding to differences based on thesensitivity curves PD11, PD12, PD13, and PD14 in FIG. 30. The horizontalaxis represents the wavelength (nm) while the vertical axis representsthe differential signal (arbitrary unit). Moreover, FIG. 32 shows astretched version of FIG. 31. As shown in FIGS. 31 and 32, sensitivityP3 to the ultraviolet light C can be figured out by finding thedifference between the light receiving sensitivity curves of the lightreceiving elements 100 and 200; that is, P3=(PD11−PD12). Moreover,sensitivity P2 to the ultraviolet lights B and C can be figured out byfinding the difference between the light receiving sensitivity curves ofthe light receiving elements 100 and 300; that is, P2=(PD11−PD13).Furthermore, sensitivity P1 to the ultraviolet lights A, B, and C can befigured out by finding the difference between the light receivingsensitivity curves of the light receiving elements 100 and 500; that is,P1=(PD11−PD14).

FIG. 33 shows that the differential signals P1, P2, and P3 in FIG. 31are used to finally find only the sensitivity to each of the ranges ofthe ultraviolet lights A, B, and C. FIG. 34 is a stretched version ofFIG. 33. Since the differential signal P3 in FIG. 31 shows sensitivityonly to the ultraviolet light C, P3 is set equal to P4; that is,P4=(PD11−PD12). Thus, the sensitivity curve of the light receivingelement 200 should be subtracted from the sensitivity curve of the lightreceiving element 100. To find the sensitivity only to the ultravioletlight B, the difference between P2 and P3 should be figured out; thatis, the sensitivity curve P5 of the ultraviolet lightB=(P2−P3)=(PD12−PD13). Thus, the sensitivity curve of the lightreceiving element 300 should be subtracted from the sensitivity curve ofthe light receiving element 200. Moreover, to find the sensitivity onlyto the ultraviolet light A, the difference between P1 and P2 should befigured out; that is, the sensitivity curve P6 of the ultraviolet lightA=(P1−P2)=(PD13−PD14). Thus, the sensitivity curve of the lightreceiving element 500 should be subtracted from the sensitivity curve ofthe light receiving element 300.

By making pairs of light receiving elements out of the four lightreceiving elements and figuring out the difference between thesensitivity curves of each pair as described above, the sensitivities tothe ranges of the ultraviolet lights A, B, and C can be detectedindividually.

In the above embodiment, the optical filters 4A, 14A, 24, and 44 areformed such that the sensitivities to the ranges of the ultravioletlights A, B, and C can be detected individually. However, the presentinvention is not limited to this example, and the paste substance may beformed containing other kinds of semiconductor particles. FIG. 35 showskinds of semiconductor particles that can be used for the opticalfilters. For example, in the configuration in FIG. 27, a filter withGaAs particles added thereto may be used as the optical filter 4A, and afilter with CdSe particles added thereto may be used as the opticalfilter 14A. In this case, formed is a photodetection device that issensitive only to a range from 710 nm (absorption edge wavelength ofCdSe) to 870 nm (absorption edge wavelength of GaAs).

In another example, in the configuration in FIG. 27, a filter with SnO₂particles added thereto may be used as the optical filter 4A, and afilter with ZnSe particles added thereto may be used as the opticalfilter 14A. In this case, formed is a photodetection device that servesas a blue sensor sensitive only to a range from 380 nm (absorption edgewavelength of SnO₂) to 500 nm (absorption edge wavelength of ZnSe), asshown in FIG. 35.

Furthermore, like MgZnO and the like, a ternary mixed crystal systemsuch as AlGaAs, InGaAs, or InGaN may be used, and the composition ratiomay be adjusted to adjust the bandgap. In this way, it is possible toform a photodetection device capable of detecting light in a desiredwavelength range. In addition to this, there is no need to use aninterference filter such as a dielectric multilayer mirror, andtherefore the optical filters can be prepared very easily.

As described above, for the semiconductor particles contained in thepaste substance making up the optical filters, it is possible to use: asemiconductor of a group IV element; a compound semiconductor of a groupII element and a group VI element; a compound semiconductor of a groupIII element and a group V element; a compound semiconductor of a groupIII element and a group VI element; or the like.

What is claimed is:
 1. A photodetection device including a plurality ofphotodetectors configured to detect light through photoelectricconversion, comprising at least: a first photodetector including a lightabsorption semiconductor layer at a side closer to a light receivingsurface of the first photodetector than is a photoelectric conversionregion of the first photodetector, the light absorption semiconductorlayer configured to absorb light in a wavelength range λ; and a secondphotodetector including a transmissive film at a side closer to a lightreceiving surface of the second photodetector than is a photoelectricconversion region of the second photodetector, the transmissive filmhaving no light absorption range, wherein an amount of light in thewavelength range λ is measured through computation using a signal fromthe first photodetector and a signal from the second photodetector. 2.The photodetection device according to claim 1, wherein the lightabsorption semiconductor layer is of Mg_(X)Zn_(1-X)O (0≦X<1).
 3. Thephotodetection device according to claim 1, wherein the transmissivefilm is made of a dielectric material.
 4. The photodetection deviceaccording to claim 1, wherein the photoelectric conversion regions ofthe respective first and second photodetectors have light receivingsensitivity to an ultraviolet range.
 5. The photodetection deviceaccording to claim 1, wherein the light absorption semiconductor layerand the transmissive film coincide with each other in terms of opticalpath length and interference fringe in a transmittance spectrum.
 6. Thephotodetection device according to claim 1, wherein a plurality ofphotodetectors each including the light absorption semiconductor layerare formed, including the first photodetector, and the light absorptionsemiconductor layers of the respective photodetectors are different fromeach other.
 7. The photodetection device according to claim 1, whereinbesides the first photodetector, photodetectors each including the lightabsorption semiconductor layer include at least a third photodetectorincluding a photoelectric conversion region having a different size fromthat of the first photodetector, and the amount of the light in thewavelength range λ, is calculated by use of the first, second, and thirdphotodetectors.
 8. The photodetection device according to claim 7,further comprising: a first calculation device for calculating aphotodetection signal J0 per unit light receiving area by using thefirst and third photodetectors, the photodetection signal J0 generatedby light in a wavelength range excluding the wavelength range λ; and asecond calculation device for calculating the amount of the light in thewavelength range λ by finding a difference between a product of S×J0 anda photodetection signal from the second photodetector, where S is alight receiving area of the second photodetector.
 9. The photodetectiondevice according to claim 1, wherein each of the photodetectors includesa reflection filter at a side closer to the light receiving surface ofthe photodetector than is the photoelectric conversion region of thephotodetector, the reflection filter configured to reflect visiblelight.
 10. The photodetection device according to claim 1, wherein inthe first photodetector, a second photoelectric conversion region isformed at a depth shallower than a first photoelectric conversionregion, in the second photodetector, a fourth photoelectric conversionregion is formed at a depth shallower than a third photoelectricconversion region, the first and third photoelectric conversion regionsare shorted, and the amount of the light in the wavelength range λ ismeasured through computation using a photodetection signal from thesecond photoelectric conversion region and a photodetection signal fromthe fourth photoelectric conversion region.
 11. A photodetection deviceincluding a plurality of photodetectors configured to detect lightthrough photoelectric conversion, comprising at least: a firstphotodetector including a first optical filter at a side closer to alight receiving surface of the first photodetector than is aphotoelectric conversion region of the first photodetector, the firstoptical filter configured to absorb light in a wavelength range λ; and asecond photodetector including a second optical filter at a side closerto a light receiving surface of the second photodetector than is aphotoelectric conversion region of the second photodetector, the secondoptical filter configured to absorb light in a wavelength range λ1including the wavelength range λ or having no light absorption range,wherein the first and second optical filters are formed such that aninterference fringe attributable to thicknesses of the filters is notpresent in a light transmission spectrum, and an amount of light in thewavelength range λ is measured through computation using a signal fromthe first photodetector and a signal from the second photodetector. 12.The photodetection device according to claim 11, wherein each of thefirst and second optical filters is of a mixture of a paste materialconfigured not to absorb the wavelength range λ and a corresponding oneof semiconductor powders having mutually different absorption edges. 13.The photodetection device according to claim 11, further comprising atleast a third photodetector including a third optical filter having thesame characteristics as those of the first optical filter, and aphotoelectric conversion region having a different size from that of thefirst photodetector, wherein the amount of the light in the wavelengthrange λ is calculated by use of the first, second, and thirdphotodetectors.
 14. The photodetection device according to claim 13,further comprising: a first calculation device for calculating aphotodetection signal J0 per unit light receiving area by using thefirst and third photodetectors, the photodetection signal J0 generatedby light in a wavelength range excluding the wavelength range λ; and asecond calculation device for calculating the amount of the light in thewavelength range λ by finding a difference between a product of S×J0 anda photodetection signal from the second photodetector, where S is alight receiving area of the second photodetector.